The present invention relates to a liquid crystal display device. Particularly, it relates to a liquid crystal display device having an improved viewing angle.
In recent years, liquid crystal display panels have found use in increasing applications. Quite recently, a black and white STN (Super Twisted Nematic) liquid crystal display panel has been developed.
In the case of this display panel, coloration is caused by birefringence inherent to a liquid crystal material. The coloration is removed by compensating for the birefringence with an optical phase retarder interposed between a liquid crystal cell and a polarizer.
As the method for compensating for the birefringence, there have been proposed a method wherein as an optical phase retarder a STN liquid crystal cell is used which is identical with a liquid crystal cell to be driven, a method wherein as an optical phase retarder an optically anisotropic oriented film made of a polycarbonate resin or polyvinyl alcohol resin of which inherent birefringence is the same in sign, or plus, as that of the STN liquid crystal cell, and a like method. With the latter in particular, the liquid crystal display panel (FTN liquid crystal display panel) is reduced in weight and the light transmittance thereof is improved. Further, there has also been proposed a liquid crystal display panel which uses an optical phase retarder in which two or more optically anisotropic oriented films of polycarbonate resin or polyvinyl alcohol resin of which inherent birefringence value assumes plus are superimposed one on the other so that the longitudinal axes of refractive index ellipses in the respective plains thereof would cross each other.
However, the use of only the optically anisotropic oriented film imparted with birefringence in the plane thereof by stretching or the like cannot achieve a proper compensation in oblique directions. This results in coloration and a narrow viewing angle.
Alternatively, there has recently been reported that increasing the refractive index of an optically anisotropic oriented film in the direction of the thickness thereof widens the range of proper compensation in oblique directions and, hence, widens the viewing angle (refer to NITTO GIHO, Vol. 28, No. 2, pp. 105 to 113, 1990). However, this literature only teaches the conditional expression: EQU (n.sub.x +n.sub.y)/2=n.sub.z
and, in addtion, there is a limitation that this conditional expression holds true on the assumption: EQU n.sub.x .perspectiveto.n.sub.y .perspectiveto.n.sub.z.
Consequently, this involves a problem that an optical compensator can be designed only on the basis of limited materials and limited combinations of optically anisotropic oriented films.
It is an object of the present invention to overcome the foregoing problems involved in the conventional methods and to provide a liquid crystal display device offering a wide viewing angle.
This and other objects of the invention will become apparent from the description hereinafter.